Measurement apparatus and method

ABSTRACT

A method and apparatus for extracting the contents ( 39 ) of voids ( 13 ) and/or pores present in a semiconductor device to obtain information indicative of the nature of the voids and/or pores, e.g. to assist with metrology measurements. The method includes heating the semiconductor wafer to expel the contents of the voids and/or pores, collecting the expelled material ( 41 ) in a collector, and measuring a consequential change in mass of the semiconductor wafer ( 29 ) and/or the collector ( 37 ), to extract information indicative of the nature of the voids. This information may include information relating to the distribution of the voids and/or pores, and/or the sizes of the voids and/or pores, and/or the chemical contents of the voids and/or pores. The collector may include a condenser having a temperature-controlled surface (e.g. in thermal communication with a refrigeration unit) for condensing the expelled material.

TECHNICAL FIELD

This invention relates to semiconductor device metrology.

BACKGROUND TO THE INVENTION

Microelectronic devices are fabricated on semiconductor wafers using avariety of techniques, e.g. including deposition techniques (CVD, PECVD,PVD, etc) and removal techniques (e.g. chemical etching, CMP, etc).During the fabrication process, various layers of metal and/ordielectric material are formed as a structure on a semiconductor waferbase, for example on a silicon wafer base.

The surface on which a given layer of metal or dielectric material isformed may have a number of topographical features which need to becoated by that layer. These topographical features may be due to theunderlying topography of the semiconductor wafer, or topographicalfeatures formed on a previous layer. Examples of topographical featuresthat may need to be coated include trenches and vias.

When a layer is formed on a surface having topographical features suchas a trench or a via, voids may be formed in the resulting structure.For example, when a layer of a material is deposited on a surface havinga trench, the deposited material may build up on the elongate sides ofthe trench until material on opposing sides of the trench meet along acentre-line of the trench, forming a seam, i.e. a line along which theregions of material on opposing sides of the trench contact each other.Voids may be formed within the trench under this seam. A plurality ofvoids may be distributed along the seam. Voids may be more likely tooccur where the walls of the trench are not uniformly flat, e.g. wherethere is a protruding lip along one or more upper edges of the trench.

FIG. 1 schematically shows a cross-section through a structure formed ona semiconductor wafer base. A layer of material 1, for example copper,has been deposited on a silicon wafer base 3. The deposited layer 1covers a trench 5 on the surface 7 of the silicon wafer base 3. Duringthe deposition process, material 1 has built up on the elongate sides 9of the trench 5 until material on opposing sides 9 of the trench 5 hasmet along a seam 11. A void 13 has been formed in the trench beneath theseam 11.

FIG. 2 schematically shows a cross-section through a structure formed ona semiconductor wafer base. Features in common with the arrangementshown in FIG. 1 are given the same reference numbers and descriptionthereof is not repeated. A via 15 is located within the trench 5. Whenthe layer of material 1 was deposited on the surface 7 of the siliconwafer base 3, a void 17 was formed within the via 15.

Voids such as those shown in FIGS. 1 and 2 can be detrimental tosemiconductor reliability. Such voids can give rise to electricalfailures during operation of a semiconductor device. Where there is avoid, there may be insufficient material to allow electrical conduction,thereby producing a short in the wiring of the semiconductor device.Also, voids might give rise to current going through a smallerconductive area, which may lead to heating and eventual failure of thesemiconductor device. At present, it is not possible to tell how manyvoids there are in a structure formed on a silicon wafer base, or todetermine information about any such voids, e.g. their size ordistribution within the layer or structure.

Voids formed in the structure, such as those shown in FIGS. 1 and 2, maycontain liquid or gas. The liquid or gas may be material which waspresent when the layer was deposited on the semiconductor wafer. Forexample, where a layer is deposited by plating, e.g. plating of copper,a liquid is applied to the surface to be coated. In the case of copperplating, this liquid may contain sulphuric acid. Thus, when a layer ofcopper is plated on a semiconductor wafer it is possible that voidscontaining liquids such as sulphuric acid may be present in theresulting structure.

In some circumstances, pores and/or voids are preferentially introducedinto a coating layer on a semiconductor wafer. When fabricating amicroelectronic device it is known to apply a coating layer of amaterial having a lower dielectric constant relative to the dielectricconstant of silicon dioxide, for example a layer of SiOC (carbon dopedsilicon oxide). These materials are commonly referred to as low-κ (orlow-k) materials. The dielectric constant of silicon dioxide is 3.9. Airhas a dielectric constant of approximately 1.00005. Thus, it is possibleto reduce the dielectric constant of a porous coating layer relative tothe dielectric constant of silicon dioxide by increasing the porosity ofthe coating layer, i.e. by reducing the average density of the materialin the coating layer.

Known forms of porous dielectric material used when fabricatingmicroelectronic devices include aerogels and xerogels. A xerogel is asolid formed from a gel by drying the gel with unhindered shrinkage.Xerogels can have porosity as high as 25% or greater. An aerogel isformed when solvent is removed from a wet gel under supercriticalconditions, i.e. at a temperature and pressure above the critical pointof the solvent. Due to the supercritical drying, aerogels retain thehighly porous structure that the gel had in the wet stage, resulting ina structure with a very low density and high porosity.

The porosity of such layers may be further increased through the use ofporogens. A porogen is a piece of material incorporated into thestructure of the coating layer which can subsequently be removed, e.g.by thermal treatment, to leave behind a void in the coating layer. Byartificially introducing voids into the coating layer in this manner,the porosity of the coating layer is increased and the dielectricconstant of the material is reduced.

Scanning electron microscopy techniques, Rutherford back-scatteringspectroscopy, small-angle neutron scattering (SANS) and ellipsometricporosimetry have been used to determine the mean pore size of porousdielectric materials. It is known to evaluate the pore content of aporous material based on the adsorption/condensation of an appropriateadsorptive in the pores. From the isotherm of adsorption, i.e. a plot ofthe amount of adsorbed/condensed adsorptive versus the relative pressureof the adsorptive at a constant temperature, it is possible to calculatean estimate of the porosity and the porous size distribution of theporous material. Such techniques require the porous material to beexposed to an adsorptive vapour while the porous material is in avacuum.

With porous dielectric layers, it is possible that the voids may containsolid material lining the void, liquid material or gas, for examplemoisture or some remaining porogen material.

In some circumstances, an air gap is preferentially introduced into asemiconductor device, e.g. to provide a layer of air between a coatinglayer and the semiconductor wafer. Such air gaps may contain solidmaterial, liquid material or gas, e.g. by-products of the process usedto produce the air gap.

SUMMARY OF THE INVENTION

At its most general, the present invention proposes a method andapparatus for extracting the contents of voids and/or pores present in asemiconductor device to obtain information indicative of the nature ofthe voids and/or pores, e.g. to assist with metrology measurements. Thisinformation may include information relating to the distribution of thevoids and/or pores, and/or the sizes of the voids and/or pores, and/orthe chemical contents of the voids and/or pores. The extraction stepinvolves heating the semiconductor device to expel the materialcontained in the pores and/or voids of the semiconductor device andcollecting the expelled material with a collector.

According to one aspect of the invention, there may be provided asemiconductor wafer metrology method comprising: heating a semiconductorwafer to expel material contained in voids formed in or beneath a layerdeposited on the semiconductor wafer; collecting the expelled materialin a collector, and measuring a consequential change in mass of thesemiconductor wafer and/or the collector, to extract informationindicative of the nature of the voids.

The consequential change in mass of the semiconductor device may be dueto the removal of the expelled material. The consequential change inmass of the collector may be due to the collection of the expelledmaterial. Both changes in mass can be used as indicators of the amountof material that is expelled from the semiconductor wafer when it isheated. The mass of material that is expelled from the semiconductordevice when it is heated is indicative of the nature of the voids in orbeneath the deposited layer. For example, the mass of expelled materialmay be indicative of the total size (i.e. volume) of the voids presentin the semiconductor wafer, as a greater mass may be expelled wherethere are more voids in the semiconductor wafer.

The changes in mass mentioned herein may be obtained by comparing a massmeasurements taken before the heating step with one or more massmeasurements taken during or after expulsion of the material in thevoids.

The method may have one, or to the extent that they are compatible, anycombination of the following optional features.

The semiconductor wafer may be heated by applying heat to a surface ofthe semiconductor wafer opposite to a surface on which the layer isdeposited. Heating the semiconductor wafer on a surface opposite to thesurface on which the layer is deposited may enable the collector to belocated in close proximity to the layer without the collector beingheated to the same temperature as the layer, i.e. a temperature gradientmay exist between the layer deposited on the semiconductor wafer and thecollector.

The semiconductor wafer may be heated by a heat source that is inthermal contact with the surface of the semiconductor wafer opposite tothe surface on which the layer is deposited. For example, thesemiconductor device may be heated by bringing it into contact with ahot surface, e.g. a heated plate.

The method may include the step of controllably adjusting thetemperature of the semiconductor device, e.g. using an adjustable heatsource, and measuring the change in mass of the semiconductor device dueto the expelled material and/or the change in mass of the collector dueto the collected material as a function of the temperature of thesemiconductor device. Thus, a change in mass of the semiconductor deviceand/or a change in mass of the collector may be linked to the particulartemperature, or range of temperatures, at which that change in massoccurs. The temperature at which material is expelled from thesemiconductor device will be related to the type of the material, i.e. ahigher or lower temperature may be required to expel liquid sulphuricacid from a void in the semiconductor device than is required to expelliquid water from a void in the semiconductor device. Therefore, bymeasuring the temperature or temperature range in which a material isexpelled from the semiconductor device, it may be possible to identifythe type of that material.

Measuring the change in mass of the semiconductor device and/or thechange in mass of the collector as a function of the temperature of thesemiconductor device may include measuring the mass of the semiconductordevice and/or the mass of the collector a plurality of times as thetemperature of the semiconductor device is gradually increased.Alternatively, the temperature of the semiconductor device may beincreased in a series of step changes, i.e. sharp increases intemperature, and the change in mass of the semiconductor device and/orthe change in mass of the collector may be measured between neighbouringstep changes in temperature. For example, the temperature of thesemiconductor device may be maintained at temperature T1 between timest1 and t2 and then temperature T2 between times t2 and t3. The change inmass of the semiconductor device and/or the change in mass of thecollector may be measured between times t1 and t2 and again betweentimes t2 and t3.

The expelled material may be gas or vapour. The collector may be acondenser for condensing (i.e. causing a phase change to a non-gaseousstate) the gaseous or liquid vapour expelled material. The expelledmaterial may therefore be retained in or on the condenser in non-gaseous(e.g. liquid or solid) form. The condenser may, for example, be a cooledsurface, e.g. a condensing plate.

In the step of collecting the expelled material, the condenser may bepositioned opposite to the surface of the semiconductor wafer on whichthe layer is deposited. As material is expelled from the semiconductordevice it may contact the condenser and condense, leading to thecollection of the majority or all of the expelled material in thecondenser. When the condenser is positioned opposite to the surface ofthe semiconductor device, expelled material may be condensed opposite tothe void from which it was expelled. Thus, the expelled material may beexpelled from the semiconductor device perpendicular to the layer andmay therefore contact the condenser opposite to the point on the layerfrom which it was expelled. Therefore, the locations of condensedexpelled material on the condenser may reflect, or map, the locations ofthe voids in the semiconductor device, i.e. the locations of thecondensed material may be representative of the locations of the voids.The locations of condensed material on the condenser may more accuratelyrepresent the locations of the voids when the condenser is closelyspaced from the semiconductor device. For example, the condenser may bespaced from the semiconductor device by a separation of less that onemillimeter. In some embodiments, the condenser may be in contact withthe surface of the semiconductor device.

In the step of collecting the expelled material, a vacuum may beprovided in a gap between the semiconductor device and the condenser,i.e. the semiconductor device and the condenser may be separated by agap occupied by a gas at a low pressure. The vacuum may be an extremelyhigh vacuum, i.e. a gas at a pressure of less than 10⁻¹⁰ Pa. Preferably,the vacuum is at a pressure of 0.1 Pa or less, e.g. between 10⁻⁹ Pa and0.1 Pa. Providing a vacuum in the gap between the condenser and thesemiconductor device may prevent the establishment of convectioncurrents that may otherwise be caused by the temperature gradientbetween the hotter semiconductor device and the colder condenser.Convection currents in the gap between the semiconductor device and thecondenser may move expelled material along the surface of the condenserbefore it is condensed on the condenser. If this occurs, the locationsof condensed material on the condenser may no longer be representativeof the locations of the voids in the semiconductor device. The pressureof the gas in the gap between the condenser and the semiconductor devicemay be related to the position of the condenser relative to thesemiconductor device in order to facilitate representative condensation.For example, a suitable separation of the condenser from thesemiconductor device may be determined from the expected or calculatedvelocity of the expelled material and/or the expected or calculated meanfree path of the expelled material in the gas in the gap between thecondenser and the semiconductor device. Alternatively, where theseparation of the condenser from the semiconductor device ispredetermined, a suitable pressure may be determined based on thisseparation.

The step of collecting the expelled material may include maintaining thetemperature of the condenser at a particular temperature. Thetemperature at which the condenser is maintained must be sufficientlylow to cause expelled material to be condensed when it contacts thecondenser. The particular temperature at which the condenser ismaintained may therefore be predetermined based on the type of materialthat is to be collected by the condenser. Alternatively, e.g. if thetype of material to be collected by the condenser is unknown, thetemperature of the condenser may be maintained at a particulartemperature that is suitable for condensing a range of differentmaterials.

The particular temperature at which the condenser is maintained may, forexample, be the ambient temperature. Alternatively, the particulartemperature may be sufficiently low to condense expelled material whichis a gas at ambient temperature, for example a temperature of 73 K, thetemperature of liquid nitrogen. Preferably, the temperature at which thecondenser is maintained is between 73 K and 373 K. The temperature ofthe condenser need not be uniformly maintained at a particulartemperature, i.e. the temperature of the condenser need not be uniformacross the condenser or uniform with time, provided that at all timesthe maximum temperature of any part of the condenser is below thecondensation temperature of expelled material.

The temperature of the condenser may be maintained at a particulartemperature by cooling the condenser using a refrigeration unit.However, other methods of maintaining the condenser at a particulartemperature may be used, for example using a liquefied gas to cool thecondenser.

The method may further comprise the step of measuring the mass of thecondenser as the temperature of the condenser is controllably increasedfrom an initial temperature at which the expelled material wascondensed. For example, the condenser may initially be cooled to atemperature sufficient to condense all of the material that is expelledfrom the semiconductor device. By measuring the mass of condensedmaterial on the condenser as the temperature of the condenser issubsequently controllably increased, it may be possible to investigatethe composition of the condensed material.

For example, the condensed material may comprise liquid A thatevaporates at temperature T1 and liquid B that evaporates at temperatureT2. As the temperature of the condenser is increased to a temperaturegreater than temperature T1, there will be a measurable change in themass of condensate on the condenser as liquid A evaporates. When thetemperature of the condenser reaches temperature T2, there will beanother measurable change in the mass of condensate on the condenser asliquid B evaporates. By accurately determining the temperatures at whichthese changes in mass occur, it may be possible to identify thecomposition of the material, i.e. it may be possible to identify thatthe liquid comprises liquid A and liquid B.

Controllably increasing the temperature of the condenser may includesmoothly and continuously increasing the temperature of the condenser.Alternatively, controllably increasing the temperature may includeincreasing the temperature of the condenser in a series of discrete stepchanges in temperature, i.e. a series of sharp increases in temperature.

In the step of collecting the expelled material, a surface of thecondenser on which the expelled material is condensed may be partitionedinto a plurality of separate condensing portions. Partitioning thecondenser into separate condensing portions means that material thatcondenses on one condensing portion is prevented from combining withmaterial that condenses on a neighbouring condensing portion. If thecondenser were not partitioned in this manner, it is possible that whena large amount of expelled material is condensed on the condenser,neighbouring regions of condensate may combine. If this occurred, theregions of condensed material on the condenser would no longer berepresentative of the locations of the voids in the semiconductordevice.

The size of the condensing portions fixes the effective resolution atwhich the locations of condensed material on the condenser map thelocations of voids in the semiconductor device. The smaller the size ofthe condensing portions, i.e. the more condensing portions there are ina given condenser, the better the resolution. Preferably, the size ofthe condensing portions is of the order of, or smaller than, the size ofthe voids in the semiconductor device. Furthermore, it may be possiblefor the condensing portions to exhibit capillary action to move thecollected material away from the surface of the collector.

The surface of the condenser may be partitioned into a uniform gridpattern of a plurality of separate condensing portions. A uniform gridpattern provides a uniform resolution across the condenser and thereforeincreases the uniformity of the mapping of the location of the voids inthe semiconductor device.

The surface of the condenser may be partitioned into a plurality ofseparate condensing portions by one or more raised ribs on the surface.The raised ribs on the surface may prevent condensed material inneighbouring condensing portions from combining.

In the step of collecting the expelled material, the collector may bepositioned so that the raised ribs on the surface of the condenser abutthe layer of the semiconductor device. Thus, when the raised ribs have acontrolled and uniform height above the surface of the condenser, thecondenser and semiconductor device may be accurately aligned inopposition to each other with a controlled spacing therebetween. Inaddition, the raised ribs may effectively partition the gap between thecondenser and the semiconductor device, thereby preventing convectioncurrents from being set up in the gap. In this case, it may therefore beunnecessary to provide a vacuum in the gap between the condenser and thesemiconductor device.

In the step of heating the semiconductor device to expel materialcontained in voids in the semiconductor device, the semiconductor devicemay be heated to a temperature sufficient to anneal material in thelayer deposited on the semiconductor wafer. For example, where the layeris a layer of copper, the semiconductor device may be raised to atemperature sufficient to anneal the copper. When a metal such as copperis annealed, the previously amorphous structure of the metal changes toa grain structure. It is believed that during the formation of thegrains, material previously trapped in voids within the semiconductordevice may be expelled along the grain boundaries. Thus, heating thesemiconductor device to a temperature sufficient to anneal material inthe layer may ensure that all of the material contained in voids in thesemiconductor device is expelled from the semiconductor device. It maybe necessary to raise the temperature of the semiconductor device to atemperature above the annealing temperature to ensure that all of thematerial initially present in the voids is expelled from thesemiconductor device.

However, it may not be necessary to heat the semiconductor device to theannealing temperature, as there may be other mechanisms that can lead tomaterial being expelled from voids in the semiconductor device. Forexample, in the arrangement shown in FIG. 1, a seam 11 connects the void13 to the surrounding atmosphere. Thus, if the temperature and pressureof the material in the void 13 is raised sufficiently, material in thevoid 13 may be expelled from the void 13 along the seam 11, withoutrequiring the temperature of the device to be raised to a temperaturesufficient to anneal the layer 1.

The method may further comprise the steps of locally heating a region ofthe condenser in order to evaporate any condensate in that region, andmeasuring the mass of the condensate evaporated from that region and/orthe change in mass of the condenser due to the evaporated material. Thechange in mass of the condenser provides an indirect measurement of themass of material evaporated from the condenser due to the local heating.Thus, it may be possible to determine whether there is any condensatepresent in a particular region of the condenser and if so how muchcondensate is present in that region. When the locations of condensateon the condenser are representative of the locations of voids in thesemiconductor device, localised measurements of the amount of condensatein a region of the condenser may provide information indicative of thenature of the voids in a corresponding region of the semiconductordevice. Where the semiconductor device and the condenser are closelyspaced in opposition to each other, the corresponding region of thesemiconductor device may be the region opposite to the region of thecondenser that is being heated.

Following the step of collecting the expelled material in the condenser,the semiconductor wafer may be removed from the locality of thecondenser, or the separation between the semiconductor wafer and thecondenser may be significantly increased. Alternatively, the condensermay be removed from the locality of the semiconductor device. Thecondenser may then be locally heated by locally applying heat to asurface of the condenser opposite to a surface on which the expelledmaterial is condensed. If the expelled material is condensed on a frontsurface, the condenser may be subsequently locally heated on a rearsurface.

A number of possible methods are envisaged for locally applying heat toa region of the condenser. Heat may be applied using a laser.Alternatively, heat may be applied by infrared radiation. Alternatively,heat may be applied by a resistive heating element in thermal contactwith a region of the condenser. Other known heating methods may also beused.

Different regions of the condenser may be sequentially locally heated,and the mass of condensate evaporated from each region and/or the changein mass of the condenser due to evaporated material from each region maybe measured. This process may continue until the whole or a substantialpart of the condenser has been locally heated. Thus, the locations atwhich condensate has formed on the condenser, and the amount ofcondensate formed at each of the locations, may be determined.Effectively, a map recording the location and mass of condensate formedon the condenser can be generated. When the locations at whichcondensate has formed on the condenser are representative of thelocations of voids in the semiconductor device, the measureddistribution of condensate across the condenser may effectively map thedistribution of voids across the surface of the semiconductor device.The localised measurements of the mass of condensate in each region ofthe condenser may provide information indicative of the nature of thevoids in corresponding regions of the semiconductor device. Thisinformation may include information relating to the number of voids inthe corresponding regions of the semiconductor device, or informationrelating to the size of the voids in the corresponding regions. Thus,information relating to the distribution of the voids and the sizes ofthe voids may be simultaneously extracted.

Sequentially heating different regions of the condenser may, forexample, involve moving the heat source relative to the condenser. Forexample, where heat is applied using a laser or an infrared source, thebeam of radiation may be moved to alter the region of the condenserbeing heated. Where the heating is provided by a resistive heatingelement, the resistive heating element may be movable relative to thecondenser. Alternatively, a plurality of fixed resistive heatingelements, each of which is in thermal contact with a different region ofthe condenser, may be positioned on the rear surface of the condenser.Each of these elements may be independently activatable so that a singleregion of the condenser may be heated.

Where the condenser is partitioned into a plurality of condensingportions, the regions of the condenser that are locally heated may eachcorrespond to different condensing portions. Thus, the temperature ofeach of the condensing portions may be independently increased by localheating, and the mass of condensate evaporated from each of thecondensing portions, and/or the change in mass of the condenser due tothe evaporation of material from each of the condensing portions, may beindependently measured.

The method may include controllably adjusting the temperature of aregion being locally heated and measuring the mass of the condensateevaporated from that region, and/or the change in mass of the condenserdue to the evaporated material, as a function of the temperature of thatregion. The temperature at which condensate is evaporated from a regionof the condenser is indicative of the type of the material in thatregion. Thus, by measuring the mass of the condensate evaporated from aregion, and/or the change in mass of the condenser due to the evaporatedmaterial, it is possible to investigate the composition of the condensedmaterial. The composition of the condensed material may be indicative ofthe contents of the voids in a corresponding region of the semiconductordevice from which the condensed material was originally expelled.

Controllably adjusting the temperature of a region of the condenser mayinclude smoothly and continuously increasing the temperature of thatregion. Alternatively, controllably increasing the temperature mayinclude increasing the temperature of that region in a series ofdiscrete step changes in temperature, i.e. a series of sharp increasesin temperature.

The mass of material evaporated from a given region being locally heatedmay be measured using a mass spectrometer. Thus, the mass of evaporatedmaterial may be accurately measured.

Where the mass of evaporated material is measured using a massspectrometer, the composition of the evaporated material may be analysedusing the mass spectrometer. Thus, it may be possible to accuratelydetermine both the mass and composition of the condensate present in aregion of the condenser being locally heated.

According to another aspect of the invention, there may be providedsemiconductor wafer metrology apparatus comprising: a heating portionfor heating a semiconductor wafer to expel material contained in voidsforming in or beneath a layer deposited on the semiconductor wafer, acollector for collecting the expelled material, and a mass measurementunit for measuring the consequential change in mass of the semiconductorwafer and/or the collector.

Features of the first aspect discussed above may be applicable to thesecond aspect.

The apparatus may have an alignment mechanism for aligning thesemiconductor device and the collector (e.g. condenser) opposite eachother.

The apparatus may further comprise a refrigeration unit for controlling,e.g. maintaining, the temperature of the condenser. The particulartemperature at which the condenser is maintained may, for example, bethe ambient temperature. Alternatively, the particular temperature maybe sufficiently low to condense expelled material which is a gas atambient temperature. The refrigeration unit may be adjustable so thatthe cooling of the condenser can be controllably adjusted, as explainedabove.

The apparatus may comprise a second heating portion for locally heatinga region of the condenser in order to evaporate any condensate in thatregion. The mass of condensate evaporated from the region being locallyheated may be indirectly measured by measuring the change in mass of thecondenser using the mass measurement unit. Alternatively, the device maycomprise a second mass measurement unit for directly measuring the massof condensate evaporated from the region being locally heated. Thus, itmay be possible to determine whether there is any condensate present ina particular region of the condenser and if so how much condensate ispresent in that region. When the locations of condensate on thecondenser are representative of the locations of voids in thesemiconductor device, localised measurements of the amount of condensatein a region of the condenser may provide information indicative of thenature of the voids in a corresponding region of the semiconductordevice.

The second heating portion may comprise a second heat source for locallyapplying heat to a surface of the condenser opposite to a surface onwhich the expelled material is condensed. If the expelled material iscondensed on a front surface, the condenser may be subsequently locallyheated on a rear surface. The second heat source may be a laser or asource of infrared radiation. Alternatively, the second heat source maybe a resistive heating element in thermal contact with a region of thecondenser.

The second heating portion may be movable relative to the condenser toalter the region of the condenser being locally heated. Moving theheating portion relative to the condenser may involve translationalmovement of the heating portion relative to the condenser, or it mayinvolve rotational movement of the heat source, e.g. where the heatsource is a laser the laser may be rotated to alter the region of thecondenser at which the laser beam is directed.

Alternatively, the second heating portion may comprise a plurality offixed resistive heating elements, each of which is in thermal contactwith a different region of the condenser. Each of these fixed resistiveheating elements may be individually activatable so that a single regionof the condenser may be heated. Where the condenser is partitioned intoa plurality of condensing portions, these regions may correspond todifferent condensing portions. Thus, the temperature of each of thecondensing portions may be independently increased by local heating, andthe mass of condensate evaporated from each of the condensing portionsmay be independently measurable.

The second mass measurement unit may comprise a mass spectrometer. Thus,the mass of evaporated material may be measured to a high degree ofaccuracy. The mass spectrometer may also be used to analyse thecomposition of the evaporated material. Therefore, it may be possible toaccurately determine both the mass and composition of the condensatepresent in the region of the condenser being locally heated.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 schematically shows a cross-section through a structure formed ona semiconductor wafer base;

FIG. 2 schematically shows a cross-section through a structure formed ona semiconductor wafer base;

FIG. 3 schematically shows a cross-section through a porous layer formedon a semiconductor wafer base;

FIG. 4 schematically shows a semiconductor device being heated and acondenser positioned opposite to the semiconductor device;

FIG. 5 schematically shows material being expelled from a semiconductordevice and collected by a condenser;

FIG. 6 is a schematic plot of an example temperature variation of asemiconductor device or a condenser with time;

FIG. 7 is a schematic plot of an example variation in mass of acondenser as it is heated;

FIG. 8 schematically shows a plan view of a condenser that ispartitioned into a plurality of condensing portions;

FIG. 9 schematically shows a view of a condenser that is partitionedinto a plurality of condensing portions by a plurality of raised ribs;

FIG. 10 schematically shows the step of evaporating condensed materialfrom a condenser and measuring the mass of the evaporated material;

FIG. 11 schematically shows aspects of device according to the secondaspect of the invention;

FIG. 12 schematically shows aspects of a device according to the secondaspect of the invention;

FIG. 13 schematically shows aspects of a device according to the secondaspect of the invention.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

In one embodiment of the invention, a semiconductor wafer metrologymethod is provided in which the contents of voids in a semiconductordevice which comprises a layer deposited on a semiconductor wafer areextracted. As illustrated in FIGS. 1 and 2, the voids in thesemiconductor device may be voids 13, 17 formed when the layer isdeposited on a surface having topographical features such as a trench 5or a via 15. In these cases, the voids may contain material which waspresent when the layer was deposited on the surface, e.g. a liquid usedwhen depositing a metal such as copper by plating, or a gas used whendepositing a layer by a vapour deposition technique.

Alternatively, as illustrated in FIG. 3, the voids may be voids and/orpores 19 within a porous layer 21, e.g. a porous dielectric layer,deposited on a semiconductor wafer 23, e.g. a silicon wafer. The porousdielectric layer may be SiOC (carbon doped silicon oxide), or anotherporous dielectric material used in the fabrication of semiconductordevices. These voids and/or pores 19 may be voids and/or pores whichnaturally occur in aerogel or xerogel structures. Alternatively, theymay be voids and/or pores which have been artificially introduced intothe layer 21 using porogens. In these cases, the voids may containmoisture or remaining porogen material.

Alternatively, the void may be an air gap formed between a layer ofmaterial and the semiconductor wafer. Such an air gap may contain solidmaterial, liquid material or gas, e.g. by-products of the process usedto produce the air gap.

The layer may be directly deposited onto the semiconductor wafer, as inthe arrangements illustrated in FIGS. 1 to 3. Alternatively, the layermay be deposited as part of a structure formed on the semiconductorwafer, i.e. the layer may be deposited on top of another layer ratherthan directly on the surface of the semiconductor wafer.

In this embodiment, as illustrated in FIG. 4, a semiconductor device 25which comprises a layer 27 formed on a semiconductor wafer 29 is heatedby applying heat 31 to a surface 33 of the semiconductor wafer oppositeto a surface 35 on which the layer 27 is deposited. In otherembodiments, the semiconductor device may be heated differently. Forexample, the semiconductor device may be uniformly heated. A collector37 is positioned opposite to the surface 35 of the semiconductor wafer29 on which the layer 27 is deposited. In other embodiments, thecollector may be differently positioned.

As illustrated in FIG. 5, as the semiconductor device 25 is heated,material 39 contained in voids 13 of the semiconductor device 25 isexpelled from the semiconductor device 25 as expelled material 41. Forexample, where the material 39 is initially in liquid form, it may bevaporised as the semiconductor device 25 is heated and may then beexpelled from the semiconductor device 25 in vapour form. The expelledmaterial 41 is collected by the collector 37. In this embodiment, thecollector 37 is a condenser and the expelled material 41 is collected bycondensing the expelled material 41 on the condenser to form condensate43. In other embodiments, other types of collector for collectingexpelled material may be used in place of a condenser.

As the semiconductor device 25 is heated, the change in mass of thesemiconductor device 25 due to the expelled material 41 and/or thechange in mass of the collector 37 due to the collected material 43 aremeasured to extract information indicative of the nature of the voids.For example, the information may be indicative of the number of voidspresent in the semiconductor device. Alternatively, the information maybe indicative of the sizes of the voids in the semiconductor device.

In this embodiment, the method includes the step of controllablyadjusting the temperature of the semiconductor device 25 and measuringthe change in mass of the semiconductor device 25 due to the expelledmaterial 41 and/or the change in mass of the collector 37 due to thecollected material 43 as a function of the temperature of thesemiconductor device 25. I.e. a change in mass of the semiconductordevice 25 and/or a change in mass of the collector 37 may be linked tothe particular temperature, or range of temperatures, at which thatchange in mass occurs. The temperature at which material 39 is expelledfrom the semiconductor device 25 will be related to the type of thematerial 39. Therefore, by measuring the temperature or temperaturerange in which a material 39 is expelled from the semiconductor device25, it may be possible to identify the type of that material 39.

As illustrated in FIG. 6, in an embodiment the temperature 45 of thesemiconductor device 25 is controllably adjusted by increasing thetemperature 45 in a series of step changes 47, i.e. sharp increases intemperature. In this embodiment, the temperature 45 of the semiconductordevice 25 is maintained at temperature T1 between times t1 and t2. Thetemperature 45 of the semiconductor device 25 is increased in a stepchange to temperature T2 at time t2 and is then maintained attemperature T2 between times t2 and t3. The change in mass of thesemiconductor device 25 and/or the change in mass of the collector 37are separately measured between each pair of step changes 47 intemperature, i.e. between times t1 and t2 and again between times t2 andt3. The change in mass is therefore linked to the particular temperaturerange in which that change in mass occurs. In other embodiments, thetemperature of the semiconductor device 25 may be smoothly andcontinuously increased with time, i.e. with no step changes 47 in thetemperature.

In this embodiment, as illustrated in FIGS. 4 and 5, during the step ofcollecting the expelled material 41 the condenser 37 is closely spacedfrom the semiconductor device 25. As material is expelled from thesemiconductor device 25 it will contact the condenser 37 and will becondensed, leading to the collection of the majority or all of theexpelled material 41 in the condenser 37. When the condenser 37 ispositioned opposite to the surface of the semiconductor device 25,expelled material 41 may be condensed opposite to the void 13 from whichit was expelled. Therefore, the locations of condensed expelled material43 on the condenser 37 may reflect, or map, the locations of the voids13 in the semiconductor device 25, i.e. the locations of the condensedmaterial 43 may be representative of the locations of the voids 13.

In this embodiment, to ensure that all of the expelled material 41 iscollected by the condenser 37, the condenser 37 is cooled by arefrigeration unit and its temperature is maintained at a sufficientlylow temperature to condense all of the expelled material 41.

In some embodiments, the mass of the condenser 37 may be measured as thetemperature of the condenser 37 is controllably increased from aninitial temperature at which the expelled material 41 was condensed. Thetemperature of the condenser 37 may be controllably increased in aseries of step changes 47, i.e. using a similar temperature profile tothat illustrated in FIG. 6. Thus, it may be possible to investigate thecomposition of the expelled material 41. The mass of the condenser 37may be continuously monitored as the temperature is increased, oralternatively the change in mass of the condenser 37 in a given timeperiod may be measured. For example, where the expelled material 41comprises liquid A that evaporates at a temperature between temperaturesT0 and T1 and liquid B that evaporates at a temperature betweentemperatures T2 and T3 and the condenser is heated as shown in FIG. 6,the variation in the mass of the condenser 27 with time may be as shownin FIG. 7. When the temperature of the semiconductor device 25 isincreased from T0 to T1 at time t1, liquid A evaporates and the mass ofthe condenser 37 decreases from M2 to M1. A change in mass of ΔM1 isrecorded between times t1 and t2. When the temperature of thesemiconductor device 25 is increased from T2 to T3 at time t3, liquid Bevaporates and the mass of the condenser decreases again from M1 to M0.A second change in mass of ΔM2 is recorded between times t2 and t3.Thus, it is possible to identify the evaporation temperatures of liquidsA and B within a range of temperatures and therefore it may be possibleto identify what materials they are.

As illustrated in FIG. 8, in this embodiment a surface of the condenser37 on which the expelled material 41 is condensed is partitioned into auniform grid pattern of a plurality of separate condensing portions 55.In other embodiments, other arrangements of condensing portions 55 maybe present other than a uniform grid pattern. In yet other embodiments,the surface of the condenser 37 may not be partitioned at all.

Partitioning the condenser 37 into separate condensing portions 55 meansthat material that condenses on one condensing portion 55 is preventedfrom combining with material that condenses on a neighbouring condensingportion 55. If the condenser 37 were not partitioned in this manner, itis possible that when a large amount of expelled material 41 iscondensed on the condenser 37, neighbouring regions of condensate 43 maycombine. If this occurred, the regions of condensed material 43 on thecondenser 37 would no longer be representative of the locations of thevoids 13 in the semiconductor device 25. In this embodiment, thecondenser 27 is partitioned into a plurality of separate condensingportions 55 by a plurality of raised ribs 57 on the surface of thecondenser 37, as illustrated in FIG. 9. In other embodiments, othertechniques may be used for partitioning the surface of the condenser 37.

As illustrated in FIG. 10, the method according to this embodimentincludes locally heating 59 a region 61 of the condenser 37 in order toevaporate any condensate 43 in that region. The mass of condensate 43that is evaporated from the region 61 is measured directly in thisembodiment by a second mass measurer 63. In other embodiments, the massof condensate 43 that is evaporated from the region 61 may be measuredindirectly by measuring the change in mass of the condenser 37. In thisembodiment, different regions of the condenser 37 are sequentiallyheated, e.g. by moving a source of the local heating relative to thecondenser 37, so that the mass of condensate 43 evaporated from each ofa plurality of regions 61 which make up a whole or a substantial part ofthe condenser 37 is measured. Thus, the locations at which condensate 43has formed on the condenser 37, and the amount of condensate 43 formedat each of the locations, may be determined. Effectively, a maprecording the location and mass of condensate 43 formed on the condenser37 can be generated. When the locations at which condensate 43 hasformed on the condenser 37 are representative of the locations of voids13, 17, 19 in the semiconductor device 25, the measured distribution ofcondensate 43 across the condenser 37 may effectively map thedistribution of voids 13, 17, 19 across the surface of the semiconductordevice 25. The localised measurements of the mass of condensate 43 ineach region 61 of the condenser 37 may provide information indicative ofthe nature of the voids 13, 17, 19 in corresponding regions of thesemiconductor device 25. This information may include informationrelating to the number of voids 13, 17, 19 in the corresponding regionsof the semiconductor device 25, or information relating to the size ofthe voids 13, 17, 19 in the corresponding regions.

In this embodiment, the second mass measurer 63 for measuring the massof condensate 43 locally evaporated from the condenser 37 is a massspectrometer which is also used to analyse the composition of theevaporated condensate 43. In other embodiments, other mass measuringdevices may be used to measure the mass of the evaporated condensate 43.

In another embodiment of the invention, as illustrated in FIG. 11, adevice 64 is provided for extracting the contents of voids 13, 17, 19 ina semiconductor device 25 comprising a layer 27 deposited on asemiconductor wafer 29. The device 64 has a heating portion 65 forheating the semiconductor device 25 to expel material 39 contained invoids 13, 17, 19 of the semiconductor device 25. In this embodiment, theheating portion 65 is a heat source which is brought into thermalcontact with a surface 33 of the semiconductor wafer 29 opposite to asurface 35 on which the layer 27 is deposited. For example, the heatingportion may be a heated surface. In other embodiments, the heatingportion 65 may not be in thermal contact with the semiconductor device25, i.e. heat may be transferred to the semiconductor device 25 by e.g.radiative heating instead. Also, in other embodiments the semiconductordevice 25 may be heated on a different surface or may be heateduniformly. The device 64 further has a collector 37 for collectingmaterial expelled from voids 13, 17, 19 in the semiconductor device. Inthis embodiment, the collector 37 is a condenser and the expelledmaterial 41 is collected by condensing the expelled material 41 on thecondenser to form condensate 43. In other embodiments, other types ofcollector for collecting expelled material may be used in place of acondenser.

As shown in FIG. 12, in this embodiment the semiconductor device 25 andthe condenser 37 are arranged opposite each other using a mount 90. Thesemiconductor device 25 is positioned adjacent to a base 91 of the mount90. The condenser 37 is slidably mounted in the mount 90 so that theseparation of the condenser 37 and the semiconductor device 25 can becontrollably varied. The semiconductor device 25 may have a notch, e.g.at an edge thereof, to indicate the crystal orientation of thesemiconductor device 25. The mount 90 may have a correspondingprotrusion so that the semiconductor device 25 can be positioned in themount 90 with the protrusion received in the notch of the semiconductordevice 25. For example, the protrusion may be a raised ridge. This ridgemay extend perpendicular to the base 91 of the mount 90 and thecondenser 37 may be slidable along this ridge to controllably vary theseparation between the semiconductor device 25 and the condenser 37. Inother embodiments, other known alignment devices may be used to alignthe semiconductor device 25 and condenser 37 in opposition to eachother.

In this embodiment, the heating portion 65 is adjustable so that thetemperature of the semiconductor device 25 can be controllably adjusted.

The device 64 includes a mass measurer 67 for measuring the change inmass of the condenser 37 due to the condensed material 43 to extractinformation indicative of the nature of the voids 13, 17, 19. Forexample, the information may be indicative of the number of voids 13,17, 19 present in the semiconductor device 25. Alternatively, theinformation may be indicative of the sizes of the voids 13, 17, 19 inthe semiconductor device 25. In other embodiments, the mass measurer 67may be arranged to measure the change in mass of the semiconductordevice 25 due to the expelled material 41 at the same time as, orinstead of, measuring the change in mass of the condenser 37 due to thecondensed material 43.

In this embodiment, the device 64 has a refrigeration unit 69 forcontrolling the temperature of the condenser 37. In this embodiment, therefrigeration unit provides cooled refrigerant to pipes 71 locatedadjacent to a rear surface of the condenser 37. In other embodiments,the pipes may be internal to the condenser 37. In yet furtherembodiments, other techniques for cooling the condenser 37, e.g.bringing a cooled liquid or gas into direct contact with the condenser37 may instead be use to control the temperature of the condenser 37. Inthis embodiment the refrigeration unit 69 is adjustable so that thetemperature of the condenser 37 can be varied. This can be achieved byadjusting the temperature or amount of the refrigerant in the pipes 71.

As illustrated in FIGS. 8 and 9, in some embodiments of the device asurface of the condenser 37 on which the expelled material 41 iscondensed is partitioned into a uniform grid pattern of a plurality ofcondensing portion 55 by a plurality of raised ribs 57 on the surface ofthe condenser 37. In other embodiments, other techniques of partitioningthe surface of the condenser 37 may be used, or the surface of thecondenser 37 may not be partitioned at all.

In this embodiment, the device 64 has a second heating portion 73 forlocally heating a region 61 of the condenser 37 in order to evaporateany condensate 43 in that region of the condenser 37. In this embodimentthe second heating portion 73 is a laser for directing a laser beam 75onto a surface of the condenser 37 opposite to a surface on which theexpelled material 43 is condensed in order to locally heat a region 61of the condenser 37. In other embodiments, other types of known heatingdevice may be used in place of a laser.

In this embodiment the device 64 has a mass measurer for measuring themass of any condensate 43 evaporated from the region 61 of the condenser37. In other embodiments, the mass of condensate 43 evaporated from thecondenser 37 may be indirectly measured by measuring the change in massof the condenser 37 due to the evaporated material.

In this embodiment the laser 73 is movable relative to the condenser 37,either by translational movement of the laser 77 or rotational movementof the laser to alter the angle of the laser beam 75, to vary the region61 of the condenser 37 being locally heated. Thus, the locations atwhich condensate 43 has formed on the condenser 37, and the amount ofcondensate 43 formed at each of the locations, may be determined.Effectively, a map recording the location and mass of condensate 43formed on the condenser 37 can be generated. When the locations at whichcondensate 43 has formed on the condenser 37 are representative of thelocations of voids 13, 17, 19 in the semiconductor device 25, themeasured distribution of condensate 43 across the condenser 37 mayeffectively map the distribution of voids 13, 17, 19 across the surfaceof the semiconductor device 25. The localised measurements of the massof condensate 43 in each region 61 of the condenser 37 may provideinformation indicative of the nature of the voids 13, 17, 19 incorresponding regions of the semiconductor device 25.

In this embodiment, the laser is adjustable, i.e. the power of the lasercan be adjusted, so that the temperature of the region 61 of thecondenser 37 being locally heated can be controllably adjusted.

The invention claimed is:
 1. A semiconductor wafer metrology apparatuscomprising: a heating portion for heating a semiconductor wafer to expelmaterial contained in voids formed in or beneath a layer deposited onthe semiconductor wafer; a collector for collecting the expelledmaterial, the collector being partitioned into a plurality of separatecollecting portions; and a mass measurement unit for measuring theconsequential change in mass of the semiconductor wafer and/or thecollector.
 2. Apparatus according to claim 1, wherein the heatingportion comprises a heat source in thermal contact with a surface of thesemiconductor wafer opposite to a surface on which the layer isdeposited.
 3. Apparatus according to claim 1, wherein the collector is acondenser having a temperature-controlled surface for condensing theexpelled material.
 4. Apparatus according to claim 3 comprising arefrigeration unit in thermal communication with the condenser tocontrol the temperature of the surface for condensing the expelledmaterial.
 5. Apparatus according to claim 1, wherein the surface of thecollector facing the semiconductor wafer is partitioned into a uniformgrid pattern of collecting portions by raised ribs on the surface.